2010-07-06

Opportunity Sol 515 (Spirit Sol 536)

It's the fifth of July. Jim Erickson had asked me to check out the drive results yesterday so that we'd have a coherent story going into the planning meeting today. I did that, and I come in half an hour early today, to boot, but somehow we still don't have a coherent story.

When Steve Squyres polls the RPs (and we all know how painful that can be), we're not ready. "We're still arguing," is all I say on mike. He laughs and tells us we can have a few more minutes.

Our overall plan had been to drive north to positions where we could evaluate two south-leading paths. One of these is West Path, which tries to take us west around Purgatory Ripple; the other is East Path. The first half of that drive, which took us to where we could evaluate West Path, occurred over the weekend. We were going to do the second half thisol, but -- we eventually agree -- we seem to have good enough imaging for both paths from our present position. (Steve is visibly overjoyed to hear this -- he really didn't want to spend another sol just figuring out where we'd go.)

So much for the easy part. But the real kicker is, do we go east or west? Since the part of Erebus we're heading for is southeast of us, it would seem like a no-brainer to choose East Path -- if we choose West Path, we're gambling that we'll eventually find a way to ripple-hop east, and we don't like to gamble. But the ripples to the east of our current position look more fearsome than their westward counterparts, making the gamble look more attractive.

Just as Steve gets back to us, though, we find a pretty good eastward solution. So we have an answer, albeit just in the nick of time. East Path wins.

As hard as it was to reach that answer, that was cake compared to the next part. The East Path on-ramp is fairly narrow, not much wider than the rover's wheel base, and it's "guarded" by a low ripple. The most appealing way to enter the on-ramp (the way that takes us directly across the lowest part of the ripple) puts us at an extremely poor comm heading -- possibly in the PMA exclusion zone, which we're supposed to try to avoid because it means the rover's trying to radiate a signal through the mast. (Which works, but it further reduces the quality of an already poor comm pass.)

We solve it by doing some stuff we're not supposed to do: we drive straight north until we're just west of the on-ramp, then turn in place and scoot directly backward (east) across the ripple. The bad things we're doing include crossing the ripple at an oblique angle, turning in place (Opportunity has a bad steering actuator in one wheel, which hinders turning in place), and driving backward instead of forward across the ripple (so if we should get stuck, we'll have a little less strength if we have to drive out forward rather than backward). But this puts us at a much better comm heading, which we need; and anyway, this is a relatively low ripple, so we shouldn't bog down in it. I just hope those aren't famous last words.

We still have one more problem. According to our new Rules of the Road, we're not supposed to drive more than 5m in one sol unless we're monitoring slip to ensure that we're not slipping more than 25%. With the way we've written the sequence, we can't check our slip precisely enough to meet this requirement. Our choices are to rewrite the sequence in a way that will make it less likely to correctly execute the drive, or cut the drive short, so that we won't make it onto the on-ramp -- or waive the rule.

John Callas, who's the acting project manager today, is extremely reluctant to waive the rule. "It's not how we said we were going to drive here," he points out. "If we're not going to do what we said we were going to do, we should be willing to change what we said we were going to do."

He's got a real point. Cindy Oda tries to get him to let us slide: "Can you just let us do it this way today, and tomorrow we'll do it whatever way you want to?" But I don't want to put John in that position, and he doesn't like it either: "So you've already got your hand in the cookie jar, so why not just let you have this cookie?" he grins.

Still, it looks to me as if John is feeling the pressure from the team, but he needs a face-saving way out. In the end, Jake Matijevic rides to the rescue. "This isn't the place to apply the Rules of the Road," he points out gently. "They were meant for terrain we haven't been over before. Here, we're just following along our old tracks, except at the very end, which is only a couple of meters of driving -- a distance we'd be willing to tolerate anyway." This gets John off the hook, and it has the extra added benefit of being true. So we're heading for the on-ramp at last.